The optical power of the eye is determined by the optical power of the cornea and that of the crystalline lens, with the lens providing about a third of the eye's total optical power. The lens is a transparent, biconvex structure whose curvature can be changed by ciliary muscles for adjusting its optical power so as to allow the eye to focus on objects at varying distances. This process is known as accommodation. As a result of accommodation, spherical aberration exhibited by the natural lens shifts in the negative direction.
The natural lens, however, becomes less transparent in individuals suffering from cataract, e.g., due to age and/or disease, thus diminishing the amount of light that reaches the retina. A known treatment for cataract involves removing the opacified natural lens and replacing it with an artificial intraocular lens (IOL). Although such IOLs can improve the patient's vision, they can, however, lead to the loss, or at least severe curtailment, of the eye's accommodative ability. In particular, a class of IOLs, commonly referred to as monofocal IOLs, provide a single optical power and hence do not allow accommodation. Another class of IOLs, commonly known as diffractive IOLs, provide primarily two optical powers, typically a far and a near optical power. As such, these IOLs provide only a limited degree of accommodation, commonly known as pseudoaccommodation.
Single-optic accommodative IOLs translate shape changes in the posterior capsule caused contraction and relaxation of the ciliary muscles into forward motion of the lens, thus providing a degree of accommodation. One difficulty facing such lenses is that the elasticity of the capsular bag can diminish as the capsular bag “shrink wraps” the IOL after surgery. Another difficulty is that the changes in shape of the capsular bag are produced by the tension and relaxation of the zonules, so that the mechanical force exerted on the IOL can be slight. The end result is that the degree of motion produced by single-optic accommodative IOLs is ordinarily insufficient to produce sufficient movement to create any perceptible visual change.
Dual-optic accommodative IOLs are also known that utilize the movement of two optical elements relative to one another in response to the movement of the ciliary muscles to provide a degree of continuous accommodation. However, the range of movement of the two optics of such IOLs is typically limited, thus restricting the range of viewing distance over which they provide accommodation. This in turn limits the degree of accommodation that can be provided.
Accordingly, there is a need for enhanced IOLs, and particularly improved accommodative IOLs as well as for methods of correcting vision that utilize them.